Isomerization of hydrocarbons



'of fluid fiow through the mass.

Patented Jan. 16,1945

ISOMERIZATION OF HYDROCARBON S John R. Callaway, Flshkill, and WilliamE.

Skelton, Beacon, N. Y., assignors, by mesnc assignments, to The TexasCompany, NewLYork, N. Y., a corporation of Delaware No Drawing.Application April 5, 1941, Serial No. 387,062

4 Claims.

This invention relates to the conversion of hydrocarbons and has to doparticularly with the isomerization of straight chain hydrocarbons toform isomers.

The invention broadly contemplates a continuous process of isomerizationof hydrocarbons wherein a stream of hydrocarbons undergoin conversion ispassed at relatively high velocity through a mass of solid pulverulentisomerization catalyst maintained under isomerizing conditions, thereaction time being of sufiiciently short duration to avoid substantialcracking of the hydrocarbons and deterioration of the catalyst.

More specifically, the invention contemplates isomerization of normalparaflin hydrocarbons such as normal butane by contact in the gas phasewith a granular or solid pulverulent catalyst, such as a catalystcomprising aluminum chloride, maintained under isomerizing conditionssuch that substantial conversion to isoparafiin is secured. Thehydrocarbons undergoing conversion are caused to flow in a continuousstream through the catalyst mass in the-presence of a suitable promoter,and at a velocity which is at least suflicient to maintain turbulentconditions In addition, a relatively short time of contact between thecatalyst and the hydrocarbons is employed, for example, not in excessof'about one and a half minutes and preferably in the range 30 secondsor less up to about 1 minute.

The invention is applicable to a continuous flow process ofisomerization wherein a stream of hydrocarbons undergoing conversion iscontinuously passed through a reaction zone containing a mass ofgranular or pulverulent catalyst. A stream of reaction mixture iscontinuously withdrawn from the reaction zone and passed to afractionating or separating zone wherein unreacted hydrocarbons areseparated from the isomerized hydrocarbons, and unreacted hydrocarbonsin part at least, continuously recycled through the isomerizationreaction zone.

An important object of the invention is to maintain conditions withinthe catalyst mass such that a uniformly high rate of conversion toisomerized hydrocarbons is maintained without experiencing substantialformation of cracked products and without substantial catalystdeterioration.

Ithas been found that by maintaining a sufficiently high velocity offluid flow through the catalyst mass, a uniformly high rate ofconversion to isoparai'fins can be maintained with a relatively shorttime of contact between the hydrocarbons and the catalyst and that thishigh conversion rate can be effected without substantial cracking orwithout substantial occurrence of objectionable side reactions. Byavoiding cracking or other objectionable side reactions, deteriorationof the catalyst is reduced to a, minimum, so that it remains effectiveduring continuous use over an extended period of time.

There appears to be a critical velocity below which the desired resultsare not realized and that is at relatively low velocities such thatconditions of viscous flow rather than conditions of turbulent flowexist within the reaction zone. Under relatively low velocity conditionshigh 1 ates of conversion to isoparaflin hydrocarbons may be realizedbut are accompanied by relatively high yields of cracked products with acontact time of the order of 3 minutes, for example. Moreover, underrelatively low velocity conditions a reduction in the time of contactbetween the hydrocarbons and the catalyst results in reducing the yieldof cracked products but is also accompanied by a corresponding reductionin the yield of isomerized hydrocarbons.

It has now been found, however, that by maintaining turbulent conditionsof flow within the catalyst mass a reduction in the time of contact.

while substantially reducing the yield of cracked products does notreduce the yield of isomerized hydrocarbons. In other words, whenoperating under conditions such that turbulent fluid flow exists in thereaction zone, altering the time of contact within certain limits doesnot cause any substantial variation in the yield of isomerizedhydrocarbons, but does materially affect the extent of cracking.

This is demonstrated when applying the process to the isomerization ofnormal butane in the gaseous phase by contact with a catalyst such asPorocel impregnated with aluminum chloride and effecting the contact, inthe presence of anhydrous hydrogen chloride, at a temperature of about210 to 220 F. and under a pressure of about pounds per square inchgauge. The catalyst in question is prepared by drying Porocel of 4 to 8mesh by heating in an oven at 580 F. for about 2 hours. The driedPorocel is then subjected to contact with aluminum chloride vapors atsubstantially atmospheric pressure for a period of 2 to 6 hours, at atemperature of about 440'F. The resulting impregnated catalyst has afree space of about 58% by volume when packed in a reaction tower, andcontains about 18 to 23% aluminum chloride b weight.

The catalyst is supported in a vertical reaction tower through which astream of normal butane in the gas phase is passed, the butane feedbeing heated sufficiently to maintain an average reaction temperature ofabout 210 to 220 F.

In one series of runs the linear velocity (calculated actual velocitythrough the free space of the catalyst mass) is varied from 1 to 2 feetto about 10 feet per minute while maintaining the same time of contactin each run, namely, about 2 minutes. The time of contact is maintainedcona parallel series of runs is made under umlar conditions butmaintaining a contact time of about 1 minute.

Upon comparing the yields obtained in these two series of runs it isfound that at a velocity of about 2 feet per minute the per centisobutane is about 54% by volume with a contact time of 2 minutes, butis only about 46% with a contact time of 1 minute. The yields of crackedproducts obtained are 9 and 5%, respectively, under the same conditions,the cracked products including hydrocarbons both lower and higher inmolecular weight than butanes. n the other hand, with a velocity ofabout4 to 5 feet per minute the yield of isobutane'is about 58% with either a1 minute or a 2 minute time of contact. with these higher velocities theyield of cracked product is about 12% by volume for a 2 minute contacttime and about 8% for a 1 minute contact time. However, the percentyield of isobutane continues to remain at about 58% for bothv timesof contact at lineal velocities as high as 10 feet per minute throughthe catalyst mass. Whereas, over the same velocity range the per centyield of cracked product continues to remain about 4% greater with a 2minute time of contact than with a 1 minute time of contact.

By reducing the time of contact still further asezssa Number asdetermined by the above formula has a value of about 40 and above.

By this'method of calculation it is found that in the case of a tubehaving an internal diameter of 1.5 inches packed with the previouslydescribed catalyst, the relationship between lineal velocity andModified Reynolds Number when normal while maintaining a high linealvelocity the yield of cracked products is correspondingly reduced. Thus,by employing a time of contact of seconds or less the yield of crackedproduct will not exceed about 2 or 3% by volume, while a uniformly highper cent conversion to isobutane is realized, namely, in the range about58 to 60% by volume, provided that turbulent flow conditions aremaintained within the reaction zone.

The relationship between lineal velocity and ther factors in order forturbulent flow to exist through a catalyst mass can be expressed bymeans of the formula for determination of Modified Reynolds Numbersdescribed in an article entitled Pressure drop in packed tubes" byChilton and Colburn, Industrial and Engineering Chemistry, August, 1931,vol. 23, No. 8, pages 913 to 919. The formula is as follows:

Modified Reynolds Numberwhere Dp is diameter of catalyst particles infeet;

U is velocity in feet per second of fluid based on total cross-sectionalarea of the tube;

P is density in pounds per cubic foot of fluid under the operatingconditions of temperature and pressure; 7

Z is viscosity in pounds per foot per second under the operatingconditions of temperature and pressure.

The values of Z for butanes can be determined by reference to the datain an article entitled Effect of pressure on viscosity of normal butaneand Chilton and Colburn article, turbulent flow exists in a packed tubewhen the Modified Reynolds butane is passed through the tube at about212 F. and under a pressure of about pounds per As set forth in theformula the lineal velocity necessary in order to maintain conditions ofturbulent fluid flow throughout the catalyst mass will depend upon theparticle size and shape of the catalyst as well as other factors such asthe viscosity of the hydrocarbons undergoing treatment and theconditions of temperature and pressure maintained within the reactionzone. Generally speaking, suitable catalysts-will have a free space inthe range about 45 to 65% by volume of the reactor space occupied by thecatalyst mass, and under such conditions the lineal velocities ofhydrocarbons through the catalyst mass should be in the range-4 or 5feet to 10 feet per minute and above, sufliciently high in other wordsto maintain conditions of turbulent flow within the reaction zone.

Any" granular or solid pulverulent type of isomerization catalyst may beemployed. Suitable catalysts include metallic halides such as aluminumchloride, zirconium chloride, etc, alone or supported upon supportingmaterials such as alumina, carbon, pumice, silica gel, etc. The catalystmay be of the impregnated type wherein a relatively inert supportingmaterial is impregnated with the active catalyst material.

Metallic halide catalysts are advantageously employed in conjunctionwith suitable promoters including anhydrous hydrogen halide such ashydrogen chloride.

The isomerization reaction may be effected in a single reaction zone. orin a plurality of reaction zones through which the hydrocarbonsundergoing conversion now in series. For example, the reaction may beeffected in a tower packed with the catalyst in the form of a single bedor a plurality of successive beds. On the other hand, a plurality ofseparate reaction towers may be employed.

Instead of employing series flow through a plurality of separatereaction towers, parallel flow of feed hydrocarbons may be employed.

A temperature gradient may be. maintained throughout the reaction zoneor zones such that relatively mild temperature conditions are maintainedduring the initial portion of the reaction, more severe conditions beingpermitted in the later stages of the reaction. For example, in theisomerization of normal butane a temperatureof about to 200 F. may bemaintained during the initial stage of the reaction and a temperature ofabout 220 to 240 F. in the final stage. Generallyspeaking, the preferredrange of temperature is about 210 to 220 F.

The temperature conditions throughout the rea,se7,sss 3 action zone maybe controlled byintroducing a portion of the feed hydrocarbon at anydesired ,w temperature to intermediate and successive points in thereaction zone or zones. Thus, cool or preheated charge may be introducedat these intermediate points as may be found necessary.

The reaction may also be controlled by multiple injection of thepromoter that is by injecting portions of the promoter at successivepoints or stages in the reaction zone. If desired increasing amounts ofthe promoter may be in- Jected at successive points in the reaction. Theamount of promoter added to the reaction may vary from about 0.5 to byweight of the total charge. If desired, all of the promoter employed maybe injected in the entering stream of feed hydrocarbon, in which case itmay constitute l to 3% by weight of the hydrocarbon feed.

The foregoing provisions for maintaining relatively mild reactionconditions in the initial stage or stages of the reaction may also beemployed to advantage where fresh catalyst is being employed in thereaction. Thus, when commencing an operation with a fresh catalystcharge, low temperature and hydrogen chloride concentration may beemployed, gradually increasing the temperature and hydrogen chlorideconcentration until normal operating conditions are reached.

While the isomerization of normal butane has been specifically describedit is contemplated that the process is applicable to the gas on vaporphase isomerization of other low boiling gasoline hydrocarbons includingpentane, hexane and the like.

While the employment of a specific pressure has been described inconnection with the isomerization of normal butane, nevertheless it iscontemplated that other pressures may be employed in carrying out thereaction, but such that the hydrocarbons undergoing reaction aremaintained in the gas phase.

The isomerization reaction may be effected in the presence of extraneousor diluent gases. For example, in isomerizing gasoline hydrocarbons suchas pentane, hexane, etc., it is advantageous to add substantial amountsof isobutane to the feed as a means of still further inhibitingundesired side reactions. The resulting reaction mixture can befractionated to segregate unreacted hydrocarbons and low boilinghydrocarbons which may be recycled, all or in part, through the reactionzone. In such case the recycled hydrocarbons may contribute materiallyin maintaining the desired velocity conditionsstage or if a singlereaction vessel is used it may be of tapered construction, the feedhydrocarbons entering the vessel at the point of narrowestcross-sectional area.

'A tapered reaction vessel with introduction of the charge at the pointof taper is of advantage where it is desired to reduce the initial rateof conversion, since in this way relatively high space velocity withshort contact time per unit depth of catalyst prevails near the chargeentry point. Frequently, the maximum degree of catalytic action existsin the initial stage or at the point at which the fresh feed hydrocarboncomes into contact with the catalyst and before the hydrocarbonsundergoing treatment have become diluted with products of reaction.Maintaining a relatively higher space velocity at this point thus tendsto prevent the existence of excessively high rates of conversion in theinitial stage of the reaction,

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore only such limitations should be imposedas are indicated in the appended claims.

We claim:

1. In a continuous process for isomerizing low boiling straight chainhydrocarbons wherein a stream of hydrocarbon undergoing conversion inthe gas phase is passed through a reaction zone containing a stationarymass of solid pulverulent isomerization catalyst at a temperature andunder conditions such that substantial conversion occurs, the stepcomprising maintaining a relatively high lineal velocity of hydrocarbonflow through the mass in the initial stage of the reaction zone andprogressively decreasing the lineal velocity of hydrocarbon flow as thehydrocarbons proceed through subsequent stages in the reaction zone suchthat a substantially uniform rate of conversion exists throughout thereaction zone.

2. In a continuous process for isomerizing hydrocarbons in the vaporphase, the steps which comprise passing vaporized feed hydrocarbonsthrough a reaction zone containing a stationary mass of isomerizationcatalyst maintained under conditions such that substantial isomerizationoccurs, maintaining a relatively high lineal velocity of hydrocarbonflow through the mass in the initial stage of the reaction zone, andprogressively decreasing the lineal velocity of hydrocarbon flow as thehydrocarbons proceed through subsequent stages in the reaction zone suchthat a substantially uniform rate of conversion exists throughout thereaction.

3. In a continuous process for isomerizing hydrocarbons in the vaporphase, the steps which comprise passing vaporized normal butane througha reaction zone containing a stationary mass of aluminum halideisomerization catalyst maintained in the presence of hydrogen halideunder conditions such that substantial isomerization occurs, maintaininga relatively high lineal velocity of hydrocarbon flow through the massin the initial stage of the reaction zone, and progressively decreasingthe lineal velocity of hydrocarbon flow as the hydrocarbons proceedthrough subsequent stages in the reaction zone such that a substantiallyuniform rate of conversion exists throughout the reaction zone.

4. The process according to claim 3 in which the isomerization reactionis efiected at a tem perature of about 210 to 220 F.

JOHN R. CALLAWAY. WILLIAM E. SKELTON.

